Assessment of pilot direct contact membrane distillation regeneration of
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lithium chloride solution in liquid desiccant air-conditioning systems using
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computer simulation
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Submitted to 4
Environmental Science and Pollution Research
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Hung Cong Duong1,2,*, Long Duc Nghiem2, Ashley Joy Ansari3, Thao Dinh Vu1, and Khai 6
Manh Nguyen4 7
1 School of Environmental Engineering, Le Quy Don Technical University, Hanoi, Vietnam 8
2 Centre for Technology in Water and Wastewater, School of Civil and Environmental 9
Engineering, University of Technology, Sydney, Broadway, NSW 2007, Australia 10
3 Strategic Water Infrastructure Laboratory, School of Civil Mining and Environmental 11
Engineering, University of Wollongong, Wollongong, NSW 2522, Australia 12
4 Faculty of Environmental Sciences, University of Science, Vietnam National University, 13
Hanoi 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam 14
15 16 17 18
_______________________
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* Corresponding author:
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Hung Cong Duong, email: [email protected]; Tel: +84 357 593 243 21
Abstract: Membrane distillation (MD) has been increasingly explored for treatment of various 22
hyper saline waters, including lithium chloride (LiCl) solutions used in liquid desiccant air- 23
conditioning (LDAC) systems. In this study, the regeneration of liquid desiccant LiCl solution by 24
a pilot direct contact membrane distillation (DCMD) process is assessed using computer 25
simulation. Unlike previous experimental investigations, the simulation allows to incorporate both 26
temperature and concentration polarisation effects in the analysis of heat and mass transfer through 27
the membrane, thus enabling the systematic assessment of the pilot DCMD regeneration of the 28
LiCl solution. The simulation results demonstrate distinctive profiles of water flux, thermal 29
efficiency, and LiCl concentration along the membrane under co-current and counter-current flow 30
modes, and the pilot DCMD process under counter-current flow is superior to that under co-current 31
flow regarding the process thermal efficiency and LiCl concentration enrichment. Moreover, for 32
the pilot DCMD regeneration of LiCl solution under the counter-current flow, the feed inlet 33
temperature, LiCl concentration, and especially the membrane leaf length exert profound impacts 34
on the process performance: the process water flux halves from 12 to 6 L/(m2h) while thermal 35
efficiency decreases by 20% from 0.46 to 0.37 when the membrane leaf length increases from 0.5 36
to 1.5 m.
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Keywords: membrane distillation (MD); direct contact membrane distillation (DCMD);
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polarisation effects; heat and mass transfer, liquid desiccant air-conditioning (LDAC); liquid 39
desiccant regeneration.
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Declarations 42
Ethics approval and consent to participate 43
Not applicable 44
Consent for publication 45
Not applicable 46
Availability of data and materials 47
The datasets generated and/or analysed during the current study are not publicly available due 48
to regulations of the funding but are available from the corresponding author on reasonable request.
49
Competing interests 50
The authors declare the following financial interests/personal relationships which may be 51
considered as potential competing interests: Vietnam National Foundation for Science and 52
Technology Development (NAFOSTED); Le Quy Don Technical University; University of 53
Technology, Sydney 54
Funding 55
This research is funded by Vietnam National Foundation for Science and Technology 56
Development (NAFOSTED) under the grant number 105.08-2019.08.
57
Authors' contributions 58
HCD: Conceptualisation, Methodology, Software, Formal Analysis, Resources, Writing- 59
Original Draft;
60
LDN: Conceptualisation, Methodology, Review & Editing 61
AJA: Methodology, Validation, Formal Analysis 62
TDV: Validation, Writing, Review & Editing 63
KMN: Methodology, Validation, Formal Analysis, Review & Editing 64
65 66
1. Introduction 67
Membrane distillation (MD), a hybrid thermal-driven separation process, has been increasingly 68
explored for treatment of various hyper saline waters due to its distinguishing attributes (Nguyen 69
et al. 2018; Abdelkader et al. 2019; Duong et al. 2019). In the MD process, a hydrophobic 70
microporous membrane is used to separate a saline water feed from a fresh distillate stream. Given 71
its hydrophobic nature, the membrane retains liquid water on the feed side, but allows the transfer 72
of water vapour through its pores to the other side, hence concentrating the feed water. The driving 73
force for the transfer of water in the MD process is not the hydraulic and/or osmotic pressure 74
difference but the water vapour pressure gradient induced by a temperature difference across the 75
membrane. As a result, unlike pressure-driven membrane processes, MD is less subject to salt 76
concentration of the feed water, and hence it is workable with various hyper saline waters including 77
concentrated brine from reverse osmosis (RO) desalination (Yan et al. 2017; Bindels et al. 2020), 78
diluted draw solution from forward osmosis (FO) (Nguyen et al. 2018), and liquid desiccant 79
solutions used in air-conditioning industry (Duong et al. 2019; Zhou et al. 2020; Liu et al. 2021).
80
Because only water vapour and volatile compounds are allowed to permeate through its membrane, 81
the MD process can achieve theoretically complete salt rejections, enabling the regeneration and/or 82
recovery of valuable dissolved salts in saline waters. More importantly, as a thermal-driven 83
separation technology, the MD process can be powered by low-grade waste heat or renewable solar 84
thermal energy to reduce the process energy cost. Given these notable attributes, MD has emerged 85
as an ideal candidate to be integrated into other process for treatment of hyper saline waters with 86
improved energy efficiency. One notable example can be the integration of MD into the liquid 87
desiccant air-conditioning (LDAC) process (Duong et al. 2017; Duong et al. 2018; Lefers et al.
88
2018; Zhou et al. 2020; Zhou et al. 2020).
89
LDAC is a potential game changer in advancing the air-conditioning industry to become 90
greener and more energy-efficient (Gurubalan et al. 2019; Chen et al. 2020; Salikandi et al. 2021).
91
Currently, most conventional air-conditioning systems are based on the vapour compression 92
process, whereby the air is first dehumidified by deep cooling to dew point temperature to condense 93
moisture and then reheated to achieve a desired temperature. The deep cooling and the subsequent 94
reheating of the air waste energy, rendering the conventional air-conditioning systems energy- 95
inefficient (Modi and Shukla 2018; Duong et al. 2019). On the other hand, LDAC systems 96
dehumidify and cool the air via the direct absorption of moisture into a liquid desiccant solution 97
(i.e. lithium chloride (LiCl) solution). The hygroscopic nature of the liquid desiccant solution drives 98
the moisture removal without the need for deep cooling and reheating the air; therefore, the energy 99
consumption of the LDAC systems is markedly reduced compared to that of the conventional 100
vapour-compression based air-conditioners (Gurubalan et al. 2019; Chen et al. 2020; Salikandi et 101
al. 2021).
102
Regeneration of liquid desiccant solution is a key step of the LDAC process (Duong et al. 2018;
103
Lefers et al. 2018; Zhou et al. 2020). The moisture holding capacity (i.e. hygroscopicity) of the 104
liquid desiccant solution depends on concentration and temperature. During the air 105
dehumidification of the LDAC process, moisture absorption dilutes and warms the liquid desiccant 106
solution, hence gradually reducing its hygroscopicity. To restore the liquid desiccant solution’s 107
hygroscopicity and hence the LDAC process’s air dehumidification efficiency, the diluted (i.e.
108
weak) liquid desiccant solution needs to be regenerated (i.e. reconcentrated and cooled) in a 109
regenerator. Most current LDAC systems rely on thermal evaporation for the regeneration of liquid 110
desiccant solutions (Cheng and Zhang 2013; Duong et al. 2019). This regeneration method involves 111
heating the diluted liquid desiccant solution to a high temperature prior to spraying it in counter- 112
current flow with a hot air stream in a packed bed media (Lowenstein 2008; Cheng and Zhang 113
2013; Salikandi et al. 2021). The direct contact between the hot liquid desiccant solution and the 114
air stream inevitably leads to the carry-over of desiccant droplets in the air stream, which is 115
regarded as a vexing technical issue of the thermal evaporation regeneration method (Duong et al.
116
2019; Gurubalan et al. 2019; Chen et al. 2020; Salikandi et al. 2021). Moreover, high-temperature 117
heating required for the regeneration of liquid desiccant solution in the evaporation regenerator 118
primarily contributes to the high energy consumption of the LDAC process. As a result, novel 119
regeneration methods that are resistant to desiccant carry-over and workable at mild temperature 120
are urgently needed for the realisation of LDAC systems. In this context, the MD process can be 121
tapped into given its complete salt rejection and workability with hyper saline waters at mild 122
temperature.
123
Previous experimental works have been conducted to prove the technical feasibility of MD for 124
the regeneration of liquid desiccant solutions used in the LDAC process (Duong et al. 2017; Duong 125
et al. 2018; Lefers et al. 2018; Zhou et al. 2019; Zhou et al. 2020; Zhou et al. 2020; Liu et al. 2021).
126
Most notably, Duong et al. (2017) experimentally investigated the direct contact membrane 127
distillation (DCMD) regeneration of liquid desiccant LiCl solutions and proved that the DCMD 128
process could regenerate the liquid desiccant LiCl solution of 29% without any issue of desiccant 129
carry-over at the feed temperature of 65 C. Zhou et al. (2020) systematically examined the 130
performance of vacuum membrane distillation (VMD) during the regeneration of LiCl solution.
131
Despite using short hollow fibre membranes (i.e. 0.52 m in length), the lab-scale VMD process 132
could increase the concentration of the LiCl 20% solution by 0.2% when operating in the single- 133
pass mode at the feed temperature of 65 C (Zhou et al. 2020). Particularly, the experimental results 134
demonstrated the profound impacts of LiCl solution temperature and membrane length on the 135
regeneration performance of the VMD process.
136
Previous lab-scale experimental works have demonstrated the viability of MD regeneration of 137
liquid desiccant LiCl solutions. It is, however, necessary to underline that there have been no 138
experimental investigations or simulation studies on pilot or large-scale MD regeneration of liquid 139
desiccant solutions, despite a great number of pilot MD processes experimentally demonstrated 140
and simulated for seawater desalination applications (Hitsov et al. 2015; Dong et al. 2017; Duong 141
et al. 2017; Andrés-Mañas et al. 2018; Andrés-Mañas et al. 2020). To facilitate the realisation of 142
MD regeneration of liquid desiccant solutions, pilot or large-scale studies are of vital importance.
143
Therefore, this study aims to assess a pilot MD process for regeneration of liquid desiccant LiCl 144
solution with the aid of computer simulation. Unlike previous simulations of pilot seawater MD 145
desalination, the pilot DCMD simulation model reported in this study incorporates the influences 146
of the LiCl solution hyper salinity and the negative effects of polarisation phenomena, particularly 147
the concentration polarisation, on the process mass and heat transfer. Given its flexibility and high 148
accuracy, the simulation package offers useful means to elucidate the mass and heat transfer inside 149
the pilot DCMD membrane module, thus allowing to elaborate the impacts of process operating 150
conditions and membrane module specifications on the process performance during the 151
regeneration of liquid desiccant LiCl solution.
152
2. Heat and mass transfer calculations and simulation approaches 153
2.1. Heat and mass transfer calculations 154
During the DCMD regeneration of LiCl solution, the transfer of water (i.e. mass transfer) occurs 155
simultaneously with the heat flux through the membrane. While the mass transfer directly controls 156
the moisture desorption and hence the regeneration of the LiCl solution, the heat flux through the 157
membrane is undesirable as it reduces the driving force of the regeneration process. The mass 158
transfer through the membrane is proportional to the water vapour pressure difference between the 159
two sides of the membrane, and is expressed as (Alkhudhiri et al. 2012):
160
. .
m m f m d
J C P P (1)
161
where J is water flux (kg/(m2h)); Cm is the membrane mass transfer coefficient (kg/(m2hPa)); and 162
Pm.f and Pm.d are the water vapour pressures (Pa) at the feed and distillate membrane surfaces, 163
respectively. Cm, a function of membrane characteristics and process operating conditions, is 164
calculated as below (Alkhudhiri et al. 2012):
165
2 1 / 1
8 2 3
M RT PD
P M
RT
Cm r a
(2) 166
where , , , and r are the membrane thickness (m), porosity (dimensionless), pore tortuosity 167
(dimensionless), and pore radius (m), respectively; M is the molecular weight of water (kg/mol); R 168
is the gas constant (i.e. 8.314 J/(molK)); T is the mean water vapour temperature (K) inside the 169
membrane pore; P and Pa are the total pressure and the air partial pressure (Pa) inside the membrane 170
pore; and D is the water diffusion coefficient (m2/s). The distillate water vapour pressure at the 171
membrane surface (i.e. Pm.d) can be calculated using the Antoine equation (Alkhudhiri et al. 2012):
172
.
.
3816.44
(23.1964 )
46.13
m d
m d
P exp
T
(3)
173
where Tm.d is the distillate temperature (K) at the membrane surface. On the other hand, the 174
calculation of water vapour pressure at the feed membrane surface (i.e. Pm.f) involves complex 175
functions of LiCl solution concentration and temperature at the feed membrane surface (e.g. Sm.f
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and Tm.f). More details of the water vapour pressure calculation of the LiCl solution at high 177
concentrations are provided elsewhere (Conde 2004; Duong et al. 2020).
178
During the DCMD process of the LiCl solution, in tandem with water vapour flux, heat is 179
transferred from the feed to the distillate via conduction through the membrane matrix and the 180
latent heat associated with the transferred water vapour. The heat flux (Q) through the membrane 181
is described as (Alkhudhiri et al. 2012):
182
. .
m
m f m d v
Q k T T JΔH
(4) 183
where Q is in kJ/(m2h); km is the membrane thermal conductivity (W/(mK)) and Hv is the latent 184
heat of evaporation of water (kJ/kg). The membrane thermal conductivity is a function of polymer 185
thermal conductivity (ks) and gas thermal conductivity (kg), expressed as (Alkhudhiri et al. 2012):
186
1
1
s g
m k k
k
(5)
187
The latent heat of water evaporation (i.e. Hv) is a function of the mean water vapour temperature 188
inside the membrane pore, and is calculated as (Alkhudhiri et al. 2012):
189
1.7535 2024.3 ΔHv T
(6)
190
Water flux calculation using the membrane mass transfer coefficient (Cm) in Eq. (1) involves 191
temperature and salt concentration at the membrane surfaces (e.g. Tm.f, Tm.d, and Sm.f). During the 192
DCMD process of LiCl solutions, polarisation effects cause the temperature and salt concentration 193
at the membrane surfaces different to those in the bulk feed and distillate streams (e.g. Tb.f, Tb.d, 194
and Sb.f) (Kuang et al. 2019; Anvari et al. 2020). While the bulk temperature and salt concentration 195
of the feed and distillate streams can be experimentally measured, the measurements of these 196
parameters at the membrane surfaces require complex instruments and impractical membrane 197
module designs (Kuang et al. 2019; Lokare et al. 2019). In this context, several studies have utilised 198
the process mass transfer coefficient (i.e. Km) together with the bulk feed and distillate temperature 199
and salt concentration for water flux calculation. This water flux calculation is more practical when 200
involving the measurable thermodynamic properties of the bulk feed and distillate; however, it fails 201
to incorporate polarisation effects, particularly concentration polarisation, resulting in considerable 202
deviations between the calculated and experimentally measured water flux (Duong et al. 2017;
203
Duong et al. 2018). The computer model developed for the simulation of the pilot DCMD process 204
of seawater reported by Duong et al. (2017) includes the temperature polarisation effect in water 205
flux calculation, but deliberately neglects the concentration polarisation effect given the negligible 206
impacts of seawater salinity on water flux. For the pilot DCMD regeneration of liquid desiccant 207
LiCl solutions, the hyper salinity of the feed exerts profound influences on water flux; therefore, 208
the concentration polarisation effect must be incorporated in water flux calculation together with 209
the temperature polarisation effect.
210
The simulation model built for this study incorporates both temperature and concentration 211
polarisation effects in water flux calculation and heat transfer analysis. Initially, water flux (J) is 212
calculated using the bulk thermodynamic properties of the feed and distillate, then the temperature 213
and LiCl concentration at the feed and distillate membrane surfaces (i.e. Tm.f , Sm.f, and Tm.d) are 214
calculated as below (Khayet et al. 2004; Hitsov et al. 2015):
215
m d m f
v d
f f b d b m f f b f m
h h h h
H Δ h J T h T h h T T
1
. . .
.
(7)
216
m f m d
v f
d d b f b m d d b d m
h h h h
H Δ h J T h T h h T T
1
. . .
.
(8)
217
. . exp
m f b f
S S J
k
(9)
218
where hm, hf, and hd are respectively the heat transfer coefficient across the membrane and in the 219
feed and distillate thermal boundary layers; and k are the density and the water transfer coefficient 220
of the LiCl solution feed. The heat transfer coefficient across the membrane (hm) is dependent on 221
the membrane thermal conductivity (km) and the membrane thickness (), while the calculations of 222
the heat transfer coefficients in the feed and distillate boundary layers (hf and hd) involve Nusselt 223
number (Nu), Reynolds number (Re), and Prandtl number (Pr) using the fluid thermodynamic 224
properties (e.g. density, dynamic viscosity, specific heat capacity, and cross flow velocity) and the 225
hydraulic diameter of the feed and distillate channels. Empirical equations for the calculations of 226
the thermodynamic properties of the LiCl solution feed and the distillate are provided in (Conde 227
2004). The calculated Tm.f, Tm.d, and Sm.f are then used for the calculation of J in equation (1). The 228
new calculated value is now assigned to J in the calculation of new Tm.f, Tm.d, and Sm.f in the 229
equations (7-9). This calculation process is iterated until the difference between the two 230
consecutive values of J is negligible.
231
Thermal efficiency is an important aspect of the DCMD process of LiCl solutions as the 232
regeneration step contributes over three quarters of the energy consumption of LDAC systems, and 233
the energy consumption of the DCMD process is primarily attributed to thermal energy required 234
for heating the feed stream. The thermal efficiency () of the DCMD process is evaluated using 235
the following equation (Alkhudhiri et al. 2012):
236
. .
( )
v m
v m f m d
Π J H
J H k T T
(10) 237
Besides thermal efficiency, the specific thermal energy consumption (i.e. STEC) of the DCMD 238
process of the LiCl solution is also assessed. STEC is the heating required to increase the weight 239
concentration of one volume unit of LiCl solution feed by 1%, and can be calculated as:
240
. .
3
.
( 25)
3.6 10
f in p f in
f in
m C T
STEC S V
(11)
241
where STEC is in kWh/(%m3); mfin is the feed inlet mass flow rate (kg/h); Cp is the specific heat 242
capacity of the LiCl solution feed (kJ/(kgC)); Tf.in is the feed inlet temperature of the DCMD 243
process; S is the LiCl concentration enrichment (i.e. the difference between the LiCl concentration 244
at the outlet and the inlet of the feed channel) (%); and Vf.in is the feed inlet volume flow rate (m3/h).
245
It is necessary to note that while STEC offers a practical indicator for the DCMD process energy 246
efficiency, demonstrates the proportion of the useful heat (i.e. that is associated with the transfer 247
of water) to the total heat transfer from the feed to the distillate along the membrane leaf inside the 248
DCMD membrane module. Moreover, the calculation of STEC for the DCMD process in this study 249
differs from that normally reported for seawater MD desalination applications because the main 250
product of the DCMD process in this study is the concentrated LiCl solution but not fresh water as 251
for seawater desalination.
252
2.2. Simulation approach 253
The simulation package used in this study is developed based on the descriptive mass and heat 254
transfer (DHMF) model that has been validated and reported in a previous study by Duong et al.
255
(2020). One notable feature of this simulation package is the inclusion of both temperature and 256
concentration polarisation effects in the mass and heat transfer analyses, and it allows for the 257
simulation of the DCMD process of the LiCl solution under two flow modes: co-current and 258
counter-current flow (Fig. 1). Details about the DHMF model and the calculation of heat and mass 259
flux through each membrane area under the two flow modes can be found in the previous study by 260
Duong et al. (2020).
261
262
Fig. 1. Schematic diagram of two incremental membrane areas along the DCMD module under 263
the (A) co-current and (B) counter-current flow mode.
264
The calculation algorithms of the DCMD process with the LiCl solution feed are illustrated in 265
Fig. 2 and Fig. 3 for co-current and counter-current flow mode, respectively. The inputs of the 266
calculation algorithms are the temperature, concentration, and mass flow rate of the LiCl solution 267
feed and distillate respectively at the feed and distillate inlets (e.g. Tf.in, Sf.in, mf.in, Td.in, and md.in).
268
The calculation starts from the feed inlet end (i.e. x0 = 0) and finishes at the feed outlet end (i.e. xn
269
= L) of the DCMD module. For co-current flow, the initial parameters of the feed and distillate 270
streams are readily available. On the other hand, for counter-current flow, initial guesses of the 271
mass flow and temperature of the distillate at the outlet (i.e. md.out and Td.out) are required (Fig. 3).
272
273
Fig. 2. Calculation algorithm of the DCMD simulation for co-current flow.
274
275
Fig. 3. Calculation algorithm of the DCMD simulation for counter-current flow.
276
The specifications of the membrane leaf and feed and distillate channels of the pilot DCMD 277
membrane module provided by AquaStill (Sittard, The Netherlands) (Hitsov et al. 2017) were used 278
for the simulation in this study. These specifications include membrane pore radius, membrane 279
porosity, membrane thickness, feed and distillate channel width and depth, and the membrane leaf 280
length. Unless otherwise stated, their default values are provided in Table 1. The pilot DCMD 281
membrane module used in (Hitsov et al. 2017) had six feed and six distillate channels, but they 282
were parallel. Thus, for simplicity the pilot DCMD membrane module simulated in this study is 283
composed of one feed and one distillate chanells with the same specifications.
284
Table 1. Specifications of the membrane leaf and flow channels of the pilot DCMD membrane 285
module 286
Membrane specifications
Pore radius (m) 0.15
Membrane porosity () 0.76
Membrane thickness (m) 92
Feed and distillate channels
Channel width (m) 0.4
Channel depth (m) 0.002
Channel length (m) 1.5
3. Results and discussions 287
3.1. Mass and heat transfer through the membrane inside the module 288
In the pilot DCMD regeneration of liquid desiccant LiCl solution, the flow mode exerts decisive 289
influence on the heat and mass transfer through the membrane. As demonstrated in Fig. 4, co- 290
current and counter-current modes result in two different feed and distillate temperatures and water 291
flux profiles inside the membrane module. Under the co-current flow mode, from the inlet to the 292
outlet of the membrane module the membrane surface feed temperature (Tm.f) declines while the 293
membrane surface distillate temperature (Tm.d) increases due to heat transferred from the feed to 294
the distillate, leading to a decrease in the transmembrane water temperature difference (i.e. Tm) 295
(Fig. 4A). This decreased Tm together with the increase in the LiCl concentration along the 296
membrane results in a rapid decline in local water flux (J) inside the membrane module from the 297
inlet to the outlet. Moreover, it is noteworthy that after the membrane length of 0.9 m, negative 298
water flux is observed despite the positive Tm (>10 C) (Fig. 4A). This finding confirms that the 299
actual driving force for water transfer through the membrane in MD is the transmembrane water 300
vapour pressure (i.e. Pm), not the transmembrane water temperature difference (Tm). After the 301
membrane length of 0.9 m, Tm.f remains markedly higher than Tm.d; however, the water vapour 302
pressure at the feed membrane surface is lower than that at the distillate membrane surface due to 303
the hyper salinity of the LiCl solution. As a result, reverse water flux from the distillate to the LiCl 304
solution feed occurs after the membrane length of 0.9 m (Fig. 4A).
305
On the other hand, the feed and distillate temperatures at the membrane surfaces and in the bulk 306
streams linearly decrease from the feed inlet to the feed outlet of the membrane module under the 307
counter-current mode (Fig. 4B). Although the temperature difference between the feed and 308
distillate membrane surfaces (Tm) remains largely constant along the membrane leaf, the local 309
water flux markedly declines from the feed inlet to the feed outlet. The declining water flux along 310
the membrane module under counter-current mode has been elucidated in the previous study by 311
Duong et al. (2020) using a lab-scale membrane module. It is noteworthy that the local water flux 312
declines at a higher rate near the feed inlet than toward the feed outlet due to the exponential 313
relation between the water vapour pressure and the temperature of solutions (Fig. 4B).
314
(A)
co-current
Fig. 4. Feed and distillate temperature at the membrane surfaces and in the bulk streams and 315
water flux along the membrane inside the module during the DCMD regeneration of the LiCl 316
20% solution under (A) co-current and (B) counter-current flow. Operating conditions: feed inlet 317
temperature (Tf.in) = 70 C, distillate inlet temperature Td.in = 25 C, feed and distillate inlet 318
circulation rate Ff.in = Fd.in = 250 L/h.
319
The discrepancy in local water flux inside the membrane module under the two operation modes 320
results in noticeably different profiles of thermal efficiency and the LiCl solution concentration 321
(Fig. 5). The thermal efficiency of the DCMD process with the LiCl 20% solution feed under both 322
operation modes is mostly below 0.5. This means that during the DCMD regeneration of the LiCl 323
20% solution feed using the pilot system, more than half of the heat transfer from the feed to the 324
distillate is due to the heat conduction through the membrane and is deemed the heat loss.
325
Furthermore, thermal efficiency under co-current mode is discernibly lower than that under the 326
counter-current mode, demonstrating that the counter-current operation is more beneficial to the 327
pilot DCMD regeneration of LiCl solution with respect to thermal efficiency. It is important to 328
stress that previous experimental studies on lab-scale DCMD regeneration of liquid desiccant LiCl 329
solutions have not investigated the process thermal efficiency.
330
The bulk LiCl concentration (i.e. Sb.f) profiles along the membrane leaf under the two flow 331
modes also clearly differ (Fig. 5). Under the co-current flow, from the feed inlet the LiCl 332
concentration steadily increases and maximizes at the membrane length of 0.9 m before gradually 333
(B)
counter-current
decreasing toward the feed outlet (i.e. 1.5 m). On the other hand, the LiCl concentration under the 334
counter-current mode progressively rises from the feed inlet to the feed outlet (Fig. 5). Indeed, 335
these LiCl concentration profiles are consistent with the water flux profiles shown in Fig. 4. The 336
decreased LiCl concentration under the co-current flow after the membrane length of 0.9 m is due 337
to the negative water flux (Fig. 4A). Moreover, the LiCl concentration at the feed outlet under the 338
counter-current flow is noticeably higher than that under the co-current mode. This also manifests 339
the advantage of the counter-current operation over the co-current one for the pilot DCMD 340
regeneration of LiCl solutions.
341
The analysis of heat and mass transfer through the membrane inside the module has revealed 342
the superiority of the counter-current to the co-current mode during the pilot DCMD regeneration 343
of the liquid desiccant LiCl solution. Thus, the counter-current mode is selected for further 344
investigations on the influences of the operating conditions and membrane length on the pilot 345
DCMD process performance discussed in the next section.
346
347
Fig. 5. Thermal efficiency and the bulk LiCl concentration along the membrane inside the 348
module during the DCMD regeneration of the LiCl 20% solution under co-current and counter- 349
current flow. Operating conditions: feed inlet temperature (Tf.in) = 70 C, distillate inlet 350
temperature Td.in = 25 C, feed and distillate inlet circulation rate Ff.in = Fd.in = 250 L/h.
351
3.2. Influences of operating conditions on the DCMD process performance 352
The key operating conditions of the pilot DCMD regeneration of LiCl solutions include the 353
feed inlet temperature, the feed and distillate circulation rate, and the inlet LiCl concentration. The 354
distillate inlet temperature has less influence on the DCMD process performance; thus, it is fixed 355
at 25 C in all simulations. The performance of the DCMD regeneration of LiCl solutions is 356
assessed using the process water flux (Jprocess), thermal efficiency (process), specific thermal energy 357
consumption (STEC), and the increase in the LiCl concentration from the inlet to the outlet (i.e.
358
S). While STEC is calculated using equation (11), Jprocess and process are the average values of 359
local water flux (J) and thermal efficiency () along the membrane leaf.
360
The simulation results reveal that it is beneficial to operate the pilot DCMD process of LiCl 361
solutions at higher feed inlet temperature and higher water circulation rates. As demonstrated in 362
Fig. 6A, elevating the feed inlet temperature boosts both Jprocess and process while substantially 363
reducing STEC of the DCMD process. Increasing feed and distillate circulation rates also favours 364
the improvement of the Jprocess and process and the reduction in STEC (Fig. 6B), but at a lower 365
extent compared to elevating the feed inlet temperature. Indeed, the benefits of operating the 366
DCMD process of LiCl solution at high feed inlet temperature and water circulation rates have 367
been proven in experimental works using lab-scale units (Duong et al. 2017; Duong et al. 2018).
368
The results reported here, however, highlight that even under the optimal feed inlet temperature 369
and water circulation rates, the pilot DCMD regeneration of LiCl solutions exhibits limited thermal 370
efficiency (i.e. process <0.5) and discernibly high STEC (i.e. 100 kWh/(%.m3)). The poor thermal 371
efficiency of the DCMD process with the LiCl solution can be attributed to the hyper salinity of 372
the LiCl solution feed.
373
Fig. 6. The process water flux (Jprocess), process thermal efficiency (process), and specific thermal 374
energy consumption (STEC) of the DCMD regeneration of the LiCl 20% solution at (A) different 375
feed inlet temperature and (B) different feed and distillate flow rate under counter-current flow.
376
Other operating conditions: distillate inlet temperature Td.in = 25 C.
377
The inlet LiCl concentration profoundly affects the performance of the DCMD process (Fig.
378
7). When the inlet LiCl concentration is elevated from 20% to 30%, Jprocess reduces by 91% from 379
(A)
(B)
5.6 to 0.5 L/(m2h), coinciding with a reduction in the process by 89% (i.e. from 0.37 to 0.04). The 380
LiCl concentration regulates not only the water vapour pressure but also the thermodynamic 381
properties (i.e. particularly the viscosity) of the feed solution, thus restraining the mass transfer 382
coefficient and water vapour flux through the membrane. Indeed, the simulation results reveal that 383
the dynamic viscosity of the LiCl solution doubles when the LiCl concentration is increased from 384
20% to 30%. Previous experimental lab-scale works have demonstrated the strong impacts of the 385
feed concentration on water flux during the DCMD regeneration of LiCl solutions (Duong et al.
386
2017; Duong et al. 2018). These impacts are even more profound for the pilot DCMD process of 387
LiCl solutions given its much longer membrane leaf compared to that used in the lab-scale units.
388
The effects of membrane leaf length on the water flux and hence the thermal efficiency of the pilot 389
DCMD process with LiCl solution will be further elucidated in section 3.3. The limited water flux 390
and poor thermal efficiency inevitably leads to the discernibly high values of STEC at higher inlet 391
LiCl concentration (Fig. 7).
392
393
Fig. 7. The process water flux (Jprocess), thermal efficiency (process), and specific thermal energy 394
consumption (STEC) of the DCMD process of the LiCl solution at different concentration. Other 395
operating conditions: feed inlet temperature (Tf.in) = 70 C, distillate inlet temperature Td.in = 25 396
C, feed and distillate inlet circulation rate Ff.in = Fd.in = 250 L/h.
397
Another important indicator for the performance of the DCMD regeneration of LiCl solutions 398
is the enrichment of LiCl in the feed (i.e. S). The three key operating conditions (e.g. feed inlet 399
temperature, water circulation rate, and the inlet LiCl concentration) exhibit different effects on S 400
(Fig. 8). The feed inlet temperature is proportional with S while elevating the LiCl solution 401
concentration noticeably reduces S (Fig. 8A&B). The impacts of feed inlet temperature and the 402
LiCl concentration on S appear similarly to their effects on water flux shown in Fig. 6A and Fig.
403
7. On the other hand, the feed and distillate circulation rates exert negligible impacts on S despite 404
having a linear relationship with water flux (Fig. 6B). Unlike the feed inlet temperature and the 405
LiCl concentration, the feed and distillate circulation rates determine the retention time of the LiCl 406
solution inside the membrane module. Increasing the water circulation rates enhances water flux 407
but also shortens the retention time of the LiCl solution feed. As a result, the impacts of the feed 408
and distillate circulation rates on S seem to be neutralized (Fig. 8C).
409
(A) (B)
Fig. 8. The LiCl concentration enrichment achieved during the DCMD regeneration of the LiCl 410
solution at various (A) feed inlet temperature, (B) inlet LiCl concentration, and (C) feed and 411
distillate water circulation rates under counter-current flow. Other operating conditions: distillate 412
inlet temperature Td.in = 25 C.
413
3.3. Influences of the membrane length on the DCMD process performance 414
Unlike experimental works using lab-scale units with fix membrane module specifications, the 415
simulation in this study allows for systematically assessing the influences of membrane module 416
specifications on the performance of the pilot DCMD regeneration of LiCl solutions. One of the 417
most critical membrane module specifications for the pilot DCMD process is the membrane leaf 418
length.
419
The simulation results reveal that the pilot DCMD process with LiCl solutions is more efficient 420
when using a shorter membrane leaf (Fig. 9). The process with the shorter membrane leaf achieves 421
higher water flux and thermal efficiency but lower STEC under the same operating conditions (e.g.
422
feed inlet temperature, feed and distillate circulation rate, and inlet LiCl concentration). The 423
enhanced process performance with the shortened membrane leaf can be attributed to the increased 424
transmembrane temperature difference (i.e. Tm). For example, at the feed inlet and the distillate 425
inlet temperatures of 70 C and 25 C, feed and distillate circulation rate of 250 L/h, and the inlet 426
LiCl concentration of 20%, the average Tm of the process with the membrane length of 0.5 m and 427
1.5 m is 25.5 C and 17.1 C, respectively. Given the exponential relation between the water vapour 428
(C)
pressure and the temperature, the reduction in Tm when increasing the membrane leaf length 429
inevitably leads to the decline in water flux (Fig. 9). This decreasing water flux in turn negatively 430
affects thermal efficiency (process) and hence raises the STEC of the process.
431
The results reported here have important implications to the design of the membrane modules 432
destined for liquid desiccant air-conditioning applications. Membrane modules with longer 433
membrane leaves offer larger membrane areas for water evaporation and hence achieve a higher 434
LiCl concentration at the outlet of the membrane modules. However, the process using longer 435
membrane exhibits lower water flux and thermal efficiency as discussed above. Therefore, for the 436
pilot DCMD regeneration of liquid desiccant solutions, it is more beneficial to deploy membrane 437
modules with short membrane leaves. The process can be operated in batch mode or brine recycling 438
mode (i.e. whereby the brine leaving the membrane modules is returned to the feed tank for further 439
treatment cycles) (Duong et al. 2015; Duong et al. 2017). Operating under these modes, the LiCl 440
concentration can be achieved without compromising the water flux and thermal efficiency of the 441
pilot DCMD process.
442
443
Fig. 9. The process water flux (Jprocess), thermal efficiency (process), and specific thermal energy 444
consumption (STEC) of the pilot DCMD regeneration of the LiCl 20% solution using the 445
membrane module with different membrane length. Other operating conditions: feed inlet 446
temperature (Tf.in) = 70 C, distillate inlet temperature Td.in = 25 C, feed and distillate inlet 447
circulation rate Ff.in = Fd.in = 250 L/h.
448
4. Conclusions 449
This study assesses the pilot DCMD regeneration of liquid desiccant LiCl solution in LDAC 450
systems using computer simulation. In contrast to experimental investigations, the simulation 451
allows for the insightful evaluation of the heat and mass transfer through the membrane inside the 452
DCMD membrane module as it can incorporate both temperature and concentration polarisation 453
effects in the calculation of heat and water flux. The simulation results demonstrate that the flow 454
mode of the pilot DCMD process strongly affects the heat and mass transfer across the membrane, 455
and the counter-current flow mode is more beneficial than co-current one regarding the process 456
water flux, thermal efficiency, and LiCl concentration enrichment. Moreover, when operating the 457
pilot DCMD process of LiCl solution under the counter-current flow, the feed inlet temperature, 458
the feed LiCl concentration, and particularly the membrane leaf length are significant factors 459
governing the process performance. When increasing the membrane leaf length from 0.5 to 1.5 m, 460
the process water flux decreases by a half from 12 to 6 L/(m2h) and thermal efficiency decreases 461
by 20%. These simulation results have important implications to the design of the pilot DCMD 462
membrane modules, particularly the membrane leaf length.
463
Acknowledgements 464
This research is funded by Vietnam National Foundation for Science and Technology 465
Development (NAFOSTED) under the grant number 105.08-2019.08.
466
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